1. Field of the Invention
This invention relates generally to an analog switch and more particularly to a monolithic switch for controlling the connection of two points in an analog signal circuit.
2. Prior Art
Analog switches are employed for connecting two points in signal path to one another. Usually, such switches employ a semiconductor device which is connected between the two points. When the semiconductor device is rendered conductive, it completes a circuit between the two points, and when the device is rendered nonconductive, it opens the circuit between the two points. Semiconductor devices have a finite amount of resistance between their switching terminals while in their conducting state.
Since a switching semiconductor device is in the signal path which it is controlling, any change in the value of its resistance while it is in its conducting state will change the resistance of that signal path. Since the effective resistance of a semiconductor device changes with changes in the bias supplied thereto, any change in the amplitude of the analog signal which is being conducted therethrough should not alter this bias. However, since one of the switching terminals of the semiconductor device forms one of the terminals to which its biasing voltage is supplied, it is not possible to isolate the analog signal in the circuit being controlled from the bias supplied to the semiconductor device.
In an attempt to overcome the above mentioned problem, a switching circuit arrangement has been employed which includes a pair of MOS field effect transistors FET having their source electrodes connected together and their drain electrodes connected together in a parallel arrangement with such electrodes forming the switching terminals. By connecting the gate of one of the FET's to the positive supply of voltage and the gate of the other FET to the negative supply of voltage, any change in the amplitude of the signal in the circuit being controlled will increase the bias on one of those FET's and decrease the bias on the other FET. With an increase in the bias of one FET, its effective resistance in the circuit being controlled will decrease, and with a decrease in the bias of the other FET, its effective resistance in the circuit being controlled will increase. Since the effective resistance of the two FET's are connected in parallel, the total resistance presented to the circuit being controlled varies less than that of a single FET employed as the switching element. However, such variation of the parallel connected FET's produces an effective resistance which changes nonlinearly with changes in the amplitude of the analog signal in the circuit being controlled.
In an attempt to overcome the above mentioned problem, other known analog switching techniques attempt to maintain the bias on the switching device constant with the changes in the analog signal of the circuit being controlled. This has been accomplished, for example, by employing a resistor connected between the biasing terminals of the switching semiconductor device, such that the voltage on one of the terminals will tend to follow or tract the voltage of the other terminal, thereby maintaining the bias on the device substantially constant. However, this technique suffers from the problem of drawing a certain amount of current from the analog signal in the circuit which is being controlled to bias the switching semiconductor device. Any current which is drawn from the analog signal in the circuit which is being controlled will result in an error in that analog signal. If, for example, a resistor is connected between the source and the gate of a switching FET, the current through the biasing resistor will be drawn from the analog signal in the circuit being controlled, since it is connected to the source of the FET. That current which is drawn from the analog signal constitutes a transient loading of the signal path being controlled, since the current through the biasing resistor will charge the stray capacitance of the gate of the switching FET. Such transient loading of the signal path occurs, therefore, when the switching FET is rendered conductive.
The use of a biasing resistor also produces a transient loading problem when the switching device is rendered nonconductive. If, for example, a resistor is connected between the source and the gate of a switching FET having its source connected to a signal source and its drain connected to a load, in and which the source of the switching FET forms a common connection point between the signal source and a second load, an error current will be introduced into the second load when the switching device is rendered nonconductive. That is, when a reverse biasing signal is supplied to the device to render it nonconductive, that signal will change the potential of the common connection point between the signal source and the second load, thereby introducing an error current into the signal path between the signal source and the second load.
In order to reduce the transient loading which occurs when the switching device is rendered conductive, a relatively small biasing resistor is employed, such that the RC time constant with the stray capacitance on the gate of the FET is relatively small. To reduce the transient loading which occurs when the switching device is rendered nonconductive, a relatively large biasing resistor must be employed. Accordingly, if a particular circuit has both of the above mentioned transient problems, a compromise must be made between the use of a relatively small valued resistor and the use of a relatively large valued resistor.
The transient loading problem which occurs when the switching device is rendered nonconductive has been solved by the use of a second FET as the biasing resistance for the switching FET. In such an arrangement, the second FET has its source connected to the source of the switching FET and its drain connected to the gate of the switching FET. Transient loading of the signal path being controlled will result when the switching FET has been rendered conductive, due to the stray capacitance on its gate.
Although it is not known that other semiconductor devices have not been employed for sensing the change in the analog signal of the circuit being controlled to maintain the bias of the switching device constant, it can be appreciated that one would consider the use of a bipolar transistor to perform that function. However, if such a transistor is employed to sense a voltage differential of the analog signal in the circuit being controlled and to supply that differential to the other electrode of the biasing circuit of the switching device, a certain amount of current will be drawn from the signal path being controlled, which current will produce an error.
Accordingly, it can be appreciated that a need exist for an analog switching circuit in which the resistance of the switching element thereof in its conducting state will not vary with changes in the analog signal of the circuit being controlled. A need also exists for such a switching circuit which produces substantially zero dc loading in the signal path being controlled.